Field of the Invention
The present invention relates to a thermal recording medium. More particularly, the present invention relates to a thermal recording medium which comprises a base layer and a thermal recording layer made of a thin film of a low melting point metal which is formed on the base layer wherein a part of the thermal recording layer is heated to melt and become perforated to record information, which is excellent in printing clarity and durability of printing clarity.
Description of the Related Art
A thermal recording medium which comprises a base layer and a thermal recording layer made of a thin film of a low melting point metal formed on the base layer, wherein a part of the thermal recording layer is heated to become melted and perforated to record information is used in various fields for indicating information, such as an amount of money, a number or a date, because of good long-time stability.
This type of thermal recording medium is disclosed in Japanese Patent Kokai Publication No. 199284/1984 and comprises a substrate, a magnetic recording medium formed on the substrate, a thermal recording layer made of a thin film of a nonmagnetic metal or alloy formed on the magnetic recording medium, and a coloring layer provided between the magnetic layer and the thermal recording layer or a coloring layer and a protecting layer on the thermal recording layer, or an adhesive layer between the adjacent layers.
In general, on a surface of the substrate made of a plastic material (e.g. nylon, cellulose diacetate, cellulose triacetate, polystyrene, polyethylene, polypropylene, polyester, polyimide and polycarbonate), the magnetic layer is formed, and on the magnetic layer, the thin film of a low melting point metal (e.g. tellurium, tin, indium, aluminum, lead and zinc) is formed by vacuum deposition or plating. A part of the low melting point metal layer is melted and perforated by means of a heating means, such as a thermal head, a laser beam or hot stamping to form a visible information, such as bar codes or OCR characters. The information formed on the thermal recording medium has far better thermal stability than informations formed through a reaction of a chemical color former.
However, the conventional thermal recording medium cannot develop prints having satisfactory clarity. That is, the conventional thermal recording medium has various drawbacks that edges of printed characters are blurred, dot area of the print is small, printing density is low, and the printing area is small.
The present inventors investigated causes for such drawbacks and found the following facts.
When adhesion of the thin metal film to the base layer is weak, the character edges tend to be blurred. Namely, when the thin metal film is heated with the thermal head, a non-printing part adjacent to a printing part of the metal film is melted or peeled off from the base layer to some extent because of heating or strain caused by heating at a boundary between the non-printing part and the printing part, whereby the character edges are blurred. The reason for this may be insufficient adhesion of the metal layer to the base layer since the conventional thermal recording medium comprises a base layer made of polyacrylate, polyurethane or vinyl chloride/vinyl acetate copolymer. When the vinyl chloride/vinyl acetate is used as the base material, it is decomposed to generate hydrogen chloride at a high temperature atmosphere of 60.degree. C. or higher and corrodes the thermal recording layer.
When thermosensitivity of the recording layer is insufficient, the printing dot area becomes small or the printing density becomes low. Namely, the heat sensitivity of the recording layer has close relationship with heat conduction in a direction across the thickness of the metal film. When heat conduction in this direction is large, the heat sensitivity of the recording layer is high. Since the conventional thermal recording medium utilizes a simple metal substance of tin and the like and the thin metal film has low strength, the metal film tends to peel off or crack when it is heated with the thermal head, whereby sufficient heat sensitivity may not be obtained. In addition, as the recording medium can be heated to a temperature of 60.degree. C. or higher with the heat applied to the metal layer to melt the simple metal substance of tin having a melting point of 232.degree. C., the corrosion of metal thin film caused by the vinyl chloride/vinyl acetate copolymer used as the base material tends to be facilitated.